REFACTOR: Use the Prices API in place of Plans (#17)

Stripe has a newer API called Prices where you can create a price for any product and it can either be recurring or one-time. The easy part is existing Plans work with the Prices API by passing a Plan ID, but objects are returned in the slightly-different Prices API object format.

This commit is a refactor to the new API to handle the data in its new form, and lays the foundation for a one time payment plan to be added to any subscriptions product.

assets/javascripts/discourse/templates/admin/plugins-discourse-subscriptions-plans-index.hbs

@@ -12,7 +12,7 @@
       <td>{{plan.id}}</td>
       <td>{{plan.nickname}}</td>
       <td>{{plan.interval}}</td>
-      <td>{{plan.amount}}</td>
+      <td>{{plan.unit_amount}}</td>
       <td class="td-right">
         {{d-button
           action=(action "editPlan" plan.id)

assets/javascripts/discourse/templates/admin/plugins-discourse-subscriptions-products-show-plans-show.hbs

@@ -80,7 +80,6 @@
   </p>
 
   <div class="pull-right">
-    {{d-button label="cancel" action=(action "cancelPlan" model.plan.product) icon="times"}}
 
     {{#if model.plan.isNew}}
       {{d-button label="discourse_subscriptions.admin.plans.operations.create" action="createPlan" icon="plus" class="btn btn-primary"}}

assets/javascripts/discourse/templates/admin/plugins-discourse-subscriptions-products-show.hbs

@@ -62,11 +62,6 @@
             {{#link-to "adminPlugins.discourse-subscriptions.products.show.plans.show" model.product.id plan.id class="btn no-text btn-icon"}}
               {{d-icon "far-edit"}}
             {{/link-to}}
-            {{d-button
-              action=(route-action "destroyPlan")
-              actionParam=plan
-              icon="trash-alt"
-              class="btn-danger btn no-text btn-icon"}}
           </td>
         </tr>
       {{/each}}